Solid polymer electrolyte having an increased conductivity and solid state cell including the electrolyte

ABSTRACT

A solid polymer electrolyte having an increased conductivity is provided  luding a solution of at least one lithium salt in at least one polymer host, and wherein said solid polymer electrolyte also includes a dispersion of a lithium ion conducting solid ceramic material. A solid state electrochemical cell including the electrolyte is also provided.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto us of any royalties thereon.

FIELD OF INVENTION

The invention relates to solid polymer electrolytes having an increasedconductivity for use in solid state polymer electrolyte batteries, andto solid state batteries including the electrolytes.

BACKGROUND OF THE INVENTION

Solid polymer electrolytes (SPEs) containing dissolved metal salts havebeen proposed as alternatives to liquid electrolytes in electrochemicalsystems. There are many advantages to using a solid electrolyte, such asthe capability for high speed production of thin cells constructed in abipolar configuration. Further, the polymer electrolyte can act as amechanical barrier between the anode and cathode thereby eliminating theneed for an inert porous separator as well as acting as abinder/adhesive to move and conform to electrode volume changes duringcycling. The polymer electrolytes also allow and facilitate thefabrication of cells in any geometric shape and also provide an inherentsafety advantage over liquid electrolytes since there is no liquidcomponent in the cell to leak out if the integrity of the sealed cell isbroken.

One of the most commonly used polymer electrolytes is based on highmolecular weight polyethylene oxide (PEO). An ionically conducting solidpolymer electrolyte can be prepared by dissolving PEO and an appropriatesalt such as lithium perchlorate (LiClO₄), lithium tetrafluoroborate(LiBF₄), lithium trifluoromethanesulfonate (LiCF₃ SO₃) or lithiumhexafluoroarsenate (LiAsF₆) in a suitable volatile solvent such asacetonitrile (CH₃ CN). By solution casting, acetonitrile is removed byevaporation, leaving a free standing solid, flexible film of goodmechanical strength that contains only PEO with dissolved salt. Suchfilms are ionic conductors.

However, because of the poor ionic conductivity of these polymers ofabout 10⁻⁷ S/cm, these polymers are not practical as electrolytes forelectrochemical cells and particularly, rechargeable cells.

SUMMARY OF THE INVENTION

The general object of this invention is to provide solid polymerelectrolytes having an increased conductivity. A more particular objectof the invention is to improve the ionic conductivity of a typicalpolymer such as PEO with dissolved salt such as LiClO₄, LiBF₄, LiCF₃ SO₃or LiAsF₆ so that it can be used as an electrolyte in solid stateelectrochemical cells.

It has now been found that the aforementioned objects can be attained byincorporating particles of a solid solution of lithium germanium oxide(Li₄ GeO₄) and lithium vanadium oxide (Li₃ VO₄) and having the generalformula, Li_(3+x) Ge_(x) V_(1-x) O₄, where 0.2<x<0.8 in the PEO-lithiumsalt polymer electrolyte.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A solid solution with the composition, Li₃.6 Ge₀.6 V₀.4 O₄ is preparedby firing a 2.3 cm pellet of a mixture of 1.33 gms of lithium carbonate,0.628 gm of germanium oxide and 0.364 gram of vanadium pentoxide that ispressed to a pressure of 6800 kg and placed on a gold foil in a ceramiccrucible. The pellet is fired at 600° C. for 20 hours to evolvecarbon-dioxide followed by heating to 900° C. for 20 hours. The firedpellet is quenched in air at room temperature and ground to a finepowder and stored in an argon filled dry box having a moisture contentof less than 0.5 ppm.

The polymer electrolyte films are prepared by dissolving PEO having anaverage molecular weight of 4×10⁶, dried at 50° C. under vacuumovernight and LiCF₃ SO₃ that has been dried at 50° C. under vacuum inmolar ratio of 20:1 respectively in acetonitrile that has been distilledunder a stream of dry argon with stirring in an argon filled dry boxhaving a moisture content of less than 5 ppm. Ten weight percent of thelithium ion conducting powdered solid material Li₃.6 Ge₀.6 V₀.4 O₄ isthen added to this solution with vigorous stirring. Films are cast bypouring the solution into flat Teflon dishes. After the solvent iscompletely evaporated, free-standing films of 50 to 100 μm in thicknessare peeled from the dishes. The conductivity of the film is thenmeasured by placing the film between stainless steel blocking electrodesand measuring the conductivities using the AC impedance technique in the5 Hz to 100 kHz frequency range with an EG&G PAR Model 388Electrochemical impedance system.

The conductivity of the PEO-LiCF₃ SO₃ films prepared with only 10 weightpercent lithium ion conducting solid ceramic additive, Li₃.6 Ge₀.6 V₀.4O₄ is found to be 10 times higher than the films prepared without theadditive at 40° C. The amount of ceramic material additive contained inthe polymer electrolyte may be varied between 0 and 100 weight percent.

When the improved solid polymer electrolyte of the invention is includedin an electrochemical cell, either primary, or rechargeable, the anodeof such a cell might be lithium metal, lithium alloy, LiC₆, lithiatedgraphite, or lithiated petroleum coke. Similarly, the cathode of such acell might be LiCoO₂, Ag₂ CrO₄, CuO, Bi₂ O₃, Bi₂ Pb₂ O₅, Cr₂ O₅, Cr₃ O₈,MnO₂, Ni₃ S₂, TiS₂, FeS₂, VSe₂, NiS₂, CoS₂, V₆ O₁₃, V₂ O₅, LiNiO₂,LiMnO₂, CuF₂,(CF)_(n), CuCl₂, AgCl, Cr_(x) V_(1-x) S₂ where x has avalue from 0 to 1.

A particular solid state electrochemical cell according to the inventionincludes lithium as the anode, LiCoO₂ on an aluminum foil currentcollector as the cathode, and (PEO)₂₀ (LiCF₃ SO₃) containing 10 weightpercent Li₃.6 Ge₀.6 V₀.4 O₄ as the solid polymer electrolyte. Thecycling conditions include a temperature of 66° C. and a charge anddischarge at 0.01 mA cm⁻² rate between 4.2-2.4 V. The capacity for cycle1 is 85.9 mAhg⁻¹ ; for cycle 2 is 74.5 mAhg⁻¹ ; and for cycle 3 is 53.5mAhg⁻¹.

We wish it to be understood that we do not des0ire to be limited to theexact details of construction shown and described for obviousmodifications will occur to a person skilled in the art.

What is claimed is:
 1. A solid polymer electrolyte for use in anelectrochemical cell, said polymer electrolyte including a solution ofat least one lithium salt in at least one polymer host and wherein saidsolid polymer electrolyte also includes a solid solution of lithiumgermanium oxide and lithium vanadium oxide having the general formulaLi_(3+x) Ge_(x) V_(1-x) O₄ where x has a value between 0.2 and 0.8.
 2. Asolid polymer electrolyte according to claim 1 wherein said lithium saltis selected from the group consisting of lithium perchlorate, lithiumtetrafluoroborate, lithium trifluoromethanesulfonate, and lithiumhexafluoroarsenate, wherein said polymer host is at least one polymerselected from the group consisting of polyethylene oxide, polyacetylene,poly (alkylthiophene), polyaniline, phenylene, and phenylsulfide, andsaid lithium ion conducting solid ceramic material is a solid solutionof lithium germanium oxide and lithium vanadium oxide having the generalformula Li_(3+x) Ge_(x) V_(1-x) O₄ where x has a value between 0.2 and0.8.
 3. A solid polymer electrolyte according to claim 2 wherein saidpolymer host is polyethylene oxide, said lithium salt is lithiumtrifluoromethanesulfonate, and said lithium ion conducting solid ceramicmaterial is Li₃.6 Ge₀.6 V₀.4 O₄.
 4. A solid state electrochemical cellincluding lithium metal, lithium metal alloys, and lithium intercalatingcompounds as the anode, an electrochemically active metallic inorganiccompound as the cathode, and a solid solution of at least one lithiumsalt in at least one polymer host as the electrolyte wherein said solidpolymer electrolyte also includes a solid solution of lithium germaniumoxide and lithium vanadium oxide having the general formula Li_(3+x)Ge_(x) V_(1-x) O₄ where X has a value between 0.2 and 0.8.
 5. A solidstate electrochemical cell including a compound selected from the groupconsisting of lithium metal, lithium metal alloy, LiC₆, lithiatedgraphite and lithiated petroleum coke as the anode, a compound selectedfrom the group consisting of LiCoO₂, Ag₂ CrO₄, CuO, Bi₂ O₃, Bi₂ Pb₂ O₅,Cr₂ O₅, Cr₃ O₈, MnO₂, Ni₃ S₂, TiS₂, FeS₂, VSe₂, NiS₂, CoS₂, V₆ O₁₃, V₂O₅, LiNiO₂, LiMnO₂, CuF₂,(CF)_(n), CuCl₂, AgCl, and Cr_(x) V_(1-x) S₂where x has a value of 0 to 1 as the cathode, and a solid solution of atleast one lithium salt selected from the group consisting of lithiumperchlorate, lithium tetrafluoroborate, lithiumtrifluoromethanesulfonate, and lithium hexafluoroarsenate in at leastone polymer host selected from the group consisting of polyethyleneoxide, polyacetylene poly(alkylthiophene), polyaniline, phenylene, andphenylsulfide as the solid polymer electrolyte and wherein said solidpolymer electrolyte also includes a dispersion of a lithium ionconducting solid ceramic material including a solid solution of lithiumgermanium oxide and lithium vanadium oxide having the general formulaLi_(3+x) Ge_(x) V_(1-x) O₄ where x has a value between 0.2 and 0.8.
 6. Asolid state electrochemical cell according to claim 5 wherein the anodeis lithium, the cathode is LiCoO₂, the polymer host is polyethyleneoxide, the lithium salt is trifluoromethanesulfonate, and the lithiumion conducting solid ceramic material is Li₃.6 Ge₀.6 V₀.4 O₄.